Low resistance measurement

There are applications where measurement of low resistances are needed. Measuring low resistance helps identify resistance elements that have increased above acceptable values. How you measure low resistances below one ohms? Most multimeters aren’t accurate below 1 ohm. Occasionally, you’ll need to measure electronic parts with very low resistances, such as wires, switches, current-sense resistors, fuses, relays, or igniters. In electronics circuits maintenance Periodic tests are made to evaluate an initial condition or to identify unexpected changes in the measured values, and the trending of this data helps indicate and may forecast possible failure conditions. Excessive changes in measured values point to the need for corrective action to prevent a major failure. Industries that consume vast amounts of electrical power must include low resistance ohmmeter measurements in their maintenance operations. Also in applications that use batteries, measurement of the internal resistance of a battery can be used to test the condition of the battery.

If you need to measure the low resistance often, it is a good idea to invest to a special four-wire or low-resistance ohmmeter. A milliohm meter, also known as a low resistance ohmmeter or micro-ohmmeter, is an instrument designed to measure very low electrical resistances, typically in the milliohm (mΩ) range. Low resistance ohmmeters are capable of measuring very low resistance values in circuits. They can measure down to fractions of micro-ohms for extreme accuracy. This is especially important for switch and contact breaker resistance, static control circuits, aircraft frame bonds, and many more applications. Milliohm and micro-ohm meters are precision test instruments capable of measuring very low resistance values in circuits, in many cases down to fractions of micro-ohms. Milliohm meters that use four-wire measurement method are essential tools in various industries for quality control, electrical engineering, and maintenance tasks where low resistance accuracy is crucial.

At measurements below one ohm it is important to use test equipment that will minimize errors introduced by the test lead resistance and/or contact resistance between the probe and the material being tested. The main technique to measure low resistance reliably are the use of a four-wire Kelvin testing. The main advantage of the four-wire measurement is that it eliminates any effect of the fixture resistance (leads) to obtain the precise resistance value of the UUT. The 4-wire Kelvin resistance method not only yields milliohm or microohm sensitive measurements, but eliminates the effects of any incidental resistance introduced by test leads or test fixtures.

The advantages of four-wire measurement come at a price. First, the test system requires twice the number of test points typically required for 2-wire resistance measurements, which greatly increases equipment cost. Second, the test fixture must use two wires for each pin on the mating connector, one for current sourcing and one for voltage sensing. This adds cost and complexity to the test fixture.

The milliohm meter includes a constant current source that passes a known current through the test specimen. This current is usually in the range of milliamperes to amperes, depending on the resistance being measured. The voltage drop across the test specimen is measured using a high-precision voltmeter. Since the voltage sensing is done using separate leads, the measurement is unaffected by the resistance of the current-carrying leads.

A “4-wire” or Kelvin measurement technique for low resistance is illustrated in figure 1.

This technique eliminates the effects of test equipment lead and probe resistance. A current of known value from the current source is forced to flow through the test resistance RDUT. A voltmeter is used to measure (sense) the drop across the resistor INSIDE the current forcing connections.

In some applications instead of constant current flowing in one direction, an alternating current signal is used for measurement (used for example in battery internal resistance testing).

If you want to do measurement without buying a new meter, you can reasonably measure down to 10 milliohms or less with a common multimeter in millivolts mode and some extra circuitry you can build. The methods used for the measurement of low resistances with multi-meter are based on ammeter Voltmeter Method: Connecting voltmeter across the load and feeding a know accurate current (like 1A) through the load using separate wires.

Get the Best Low Resistance Measurements Possible

Those techniques work for resistances. But is is possible to measure internal resistance of battery?

What is Internal Resistance (IR) and how to test battery resistance? There are several techniques that allow internal resistance measurement with batteries also.
The Alternating Current Internal Resistance, commonly called AC Impedance or Impedance, is usually tested with an Impedance Analyzer.

Direct Current Internal Resistance, DCIR or DCR can be measured with a battery tester by applying a low current followed by higher current on the battery within a short period, and then record the changes of battery voltage ΔV and current ΔI. Direct Current Internal Resistance, DCIR or DCR can be measured with a battery tester by applying a low current followed by higher current on the battery within a short period, and then record the changes of battery voltage ΔV and current ΔI. IEC61960 is one standard that describes how the DC resistance can be measured.

The AC Impedance and DCIR both indicate battery’s power characteristics while DCIR is more popularly used by battery manufacturers since the test is simple and the results are reliable.

Sources:

https://www.robotroom.com/Measuring-Low-Resistances.html

https://www.instrumart.com/categories/6209/low-resistance-ohmmeters

https://jmtest.com/megger-digital-low-resistance-ohmmeter/

https://www.instrumart.com/assets/Megger-Low-Resistance-Guide.pdf

https://www.kritester.com/new/low-resistance-test.html

https://info.powershield.com/blog/battery-internal-ohmic-measurements-part-2

https://wiki.analog.com/university/courses/tutorials/alm-milli-ohm-meter

https://www.electronicsforu.com/electronics-projects/milliohm-meter

Measuring very small resistances, A Milliohm-meter

https://wiki.analog.com/university/courses/tutorials/alm-milli-ohm-meter

Four-terminal sensing

https://en.wikipedia.org/wiki/Four-terminal_sensing